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ii                               CANINES AND  PREMOLARS                            49
" horn " of the USTarwhal is another modification of an incisor, as
are the tusks of Elephants. Among the Lemurs the incisors are
denticulate, and serve to clean the fur in a comb-like fashion.
This is markedly the case in Galeopithecus. The incisors are
sometimes totally absent, as in the Sloths, sometimes partially
absent, as in many Artiodactyles, where the lower incisors bite
against a callous pad in the upper jaw, in which no trace of
incisors has been found.
Canine teeth are present in the majority of mammals, but
are absent without a single exception from the jaws of the
Ilodentia. The canine tooth of the upper jaw is that tooth
which comes immediately after the suture dividing the pre-
maxillary from the maxillary bone. The canines are as a rule
simple conical teeth, with but a single root; indeed they
resemble what we may presume to have been the first kind of
tooth developed in mammals. In this they resemble also as a
general rule the foregoing incisors. But instances are known
where the canines are implanted by two roots. This is to be
seen in Trlconodon, in the pig ^Hyotheriuttn> in the Mole and
some other Insectivores, and in G-aleopifhecus, where the incisors
also may be thus -implanted in the jaw. Furthermore, the
simple condition of the crown of the tooth may be departed from.
This is the case with a Fruit Bat belonging to the genus Ptera-
lopex. In the more primitive Mammalia it is common to find
no great difference between the canines and incisors ; such is the
case with the early Ungulate types of Eocene times, such as
Kiphodon. In modern mammals, however, especially among the
Camivora, the canines tend to become larger and. stronger than
the incisors, and in. some of the Cats and in the Walrus these teeth
are represented by enormous offensive tusks. It is not rare for
the canines of male animals to be larger than those of their
mates. There are also cases sxich as the Musk-deer and the
Kaiichil where the male alone possesses these teeth, but only in
the upper jaw. The teeth which follow the canines are known
o,s the grinders or cheek teeth, or more technically as premolars
arid molars. These two latter terms separate teeth which arise
at different periods, and their use will be explained later. In the
meantime it may be poiiited oxit that the cheek teeth, are the teeth
which show the greatest amount of variation in their structure;
this is shown by the number and variety of the cusps in which
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